(On the Mississippi River during the time of Samuel Clemens, A rope or rod with fathom markings was used to gauge the depth of the river. A depth of less than two fathoms was considered risky for boats to navigate across. Who ever was doing the depth measure would call out the markings by terms such as twain for two, trey for three, quad for four........ and so forth. Samuel Clemens worked on a river boat before he became a writer. when he decide to write under a pen name, he choose Mark Twain (safe passage).